Add get_sandbox_debug_log(), and use it (add behaviour similar to SANDBOX_LOG
if already exported when sandbox started). Fix get_sandbox_log() and new
get_sandbox_debug_log() to not use already exported environment variables if
they have '/' in them. Use snprintf()'s instead of strncpy()'s. More
SB_PATH_MAX fixes.

Make sure all functions used in libsandbox.c is declared static. Define
SB_STATIC in localdecls.h for this. Include sandbox_futils.c rather than
linking with its object. Hopefully this will fix bug #90153.